Universal mobile telecommunications system (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.
The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.
FIG. 1 is a block diagram illustrating network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed to provide a variety of communication services such as voice and packet data.
As illustrated in FIG. 1, the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN) and an Evolved Packet Core (EPC) and one or more user equipment. The E-UTRAN may include one or more evolved NodeB (eNB) 20, and a plurality of user equipment (UE) 10 may be located in one cell. One or more E-UTRAN mobility management entity (MME)/system architecture evolution (SAE) gateways 30 may be positioned at the end of the network and connected to an external network.
As used herein, “downlink” refers to communication from an eNB 20 to a UE 10, and “uplink” refers to communication from the UE to an eNB. The UE 10 is communication equipment carried by a user and may also be referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.
An eNB 20 provides end points of a user plane and a control plane to the UE 10. The MME/SAE gateway 30 provides an end point of a session and mobility management function for a UE 10. The eNB 20 and MME/SAE gateway 30 may be connected via an S1 interface.
The eNB 20 is generally a fixed station that communicates with a UE 10 and may also be referred to as a base station (BS) or an access point. One eNB 20 may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNBs 20.
The MME provides various functions including distributing paging messages to the eNBs 20, security control, idle state mobility control, SAE bearer control, and ciphering and integrity protection of non-access stratum (NAS) signaling. The SAE gateway host provides assorted functions including termination of U-plane packets for paging reasons and switching the U-plane to support UE 10 mobility.
The MME/SAE gateway 30 will be referred to herein simply as a “gateway” for clarity. However, it is understood that the MME/SAE gateway 30 includes both an MME and an SAE gateway.
A plurality of nodes may be connected between the eNB 20 and the gateway 30 via the S1 interface. The eNBs 20 may be connected to each other via an X2 interface and neighboring eNBs may have a meshed network structure that has the X2 interface.
FIG. 2(a) is a block diagram depicting architecture of a typical E-UTRAN and a typical gateway 30. As illustrated in FIG. 2(a), the eNB 20 may perform functions such as selection for gateway 30, routing toward the gateway during a Radio Resource Control (RRC) activation, scheduling and transmitting paging messages, scheduling and transmitting Broadcast Channel (BCCH) information, dynamic allocation of resources to UEs 10 in both uplink and downlink, configuration and provisioning eNB measurements, radio bearer control, radio admission control (RAC), and connection mobility control in LTE_ACTIVE state. In the EPC, the gateway 30 may perform functions such as paging origination, LTE-IDLE state management, ciphering the user plane, System Architecture Evolution (SAE) bearer control, and ciphering and integrity protection of Non-Access Stratum (NAS) signaling.
FIGS. 2(b) and 2(c) are block diagrams depicting the user-plane protocol and the control-plane protocol stack for the E-UMTS. As illustrated in FIGS. 2(b) and 2(c), the protocol layers may be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based upon the three lower layers of an open system interconnection (OSI) standard model that is well-known in the art of communication systems.
The physical layer, or first layer (L1), provides an information transmission service to an upper layer by using a physical channel. The physical layer is connected to a medium access control (MAC) layer located at a higher level through a transport channel, with data transferred between the MAC layer and the physical layer via the transport channel. Data is transferred via a physical channel between different physical layers, such as between the physical layer of a transmission side and the physical layer of a reception side.
The MAC layer of Layer 2 (L2) provides services to a radio link control (RLC) layer, which is a higher layer, via a logical channel. The RLC layer of Layer 2 (L2) supports the transmission of data with reliability. It should be noted that although the RLC layer is illustrated in FIGS. 2(b) and 2(c), the RLC layer is not required if the MAC layer performs the RLC functions.
The PDCP layer of Layer 2 (L2) performs a header compression function that reduces unnecessary control information. This allows efficient transmission of data utilizing Internet protocol (IP) packets, such as IPv4 or IPv6, over a radio or wireless interface that has a relatively small bandwidth.
A radio resource control (RRC) layer located at the lowest portion of the third layer (L3) is only defined in the control plane and controls logical channels, transport channels and the physical channels in relation to the configuration, reconfiguration, and release of the radio bearers (RBs). A RB signifies a service provided by the second layer (L2) for data transmission between a UE 10 and the E-UTRAN.
As illustrated in FIG. 2(b), the RLC and MAC layers are terminated in an eNB 20 on the network side and may perform functions such as scheduling, Automatic Repeat Request (ARQ), and hybrid automatic repeat request (HARQ). The PDCP layer is terminated in an eNB 20 on the network side and may perform the user plane functions such as header compression, integrity protection, and ciphering.
As illustrated in FIG. 2(c), the RLC and MAC layers are terminated in an eNB 20 on the network side and perform the same functions as for the control plane. As illustrated in FIG. 2(c), the RRC layer is terminated in an eNB 20 on the network side and may perform functions such as broadcasting, paging, RRC connection management, Radio Bearer (RB) control, mobility functions, and UE 10 measurement reporting and controlling. As illustrated in FIG. 2(c), the NAS control protocol is terminated in the MME of gateway 30 on the network side and may perform functions such as a SAE bearer management, authentication, LTE_IDLE mobility handling, paging origination in LTE_IDLE, and security control for signaling between the gateway and UE 10.
The NAS control protocol may use three different states. An LTE_DETACHED state is used if there is no RRC entity. An LTE_IDLE state is used if there is no RRC connection while storing minimal UE 10 information. An LTE_ACTIVE state is used if the RRC connection is established. Furthermore, the RRC state may be divided into two different states, such as RRC_IDLE and RRC_CONNECTED.
In RRC_IDLE state, the UE 10 may receive broadcasts of system information and paging information while the UE specifies a Discontinuous Reception (DRX) configured by the NAS and the UE has been allocated an identification (ID) which uniquely identifies the UE in a tracking area. Furthermore, no RRC context is stored in the eNB in RRC-IDLE state.
In RRC_CONNECTED state, the UE 10 has an E-UTRAN RRC connection and a context in the E-UTRAN, such that transmitting and/or receiving data to and from the eNB is possible. Furthermore, the UE 10 can report channel quality information and feedback information to the eNB.
In RRC_CONNECTED state, the E-UTRAN knows the cell to which the UE 10 belongs. Therefore, the network can transmit and/or receive data to and from the UE 10, control mobility, such as handover, of the UE, and perform cell measurements for a neighboring cell.
In RRC_IDLE mode, the UE 10 specifies the paging DRX Discontinuous Reception cycle. Specifically, the UE 10 monitors a paging signal at a specific paging occasion of every UE specific paging DRX cycle.
FIG. 3 illustrates the conventional LTE handover procedure. The UE 10 sends a measurement report to the source eNB 20 (S102). The source eNB 20 sends a handover request message with the UE 10 context to the target eNB (S104).
The target eNB 20 sends a handover request response to the source eNB (S106). The handover request response includes the new CRNTI, a portion of a handover command message and information related to random access, such as a dedicated access signature for the UE 10 to make a contention-free random access on the target cell. A signature is reserved at this time.
The source eNB 20 sends the handover command to the UE (S108). The handover command includes the new C-RNTI and information related to random access, such as the dedicated signature for the UE 10 to use.
A random access procedure is performed in the target cell after the handover command in order for the UE 10 to obtain the timing advance (TA) value. This random access procedure should be contention-free such that a signature is reserved to the UE 10 in order to avoid collision.
The UE 10 starts the random access procedure on the target eNB 20 by sending the random access preamble using a dedicated signature (S110). The target eNB 20 sends the random access response message to the UE 10 (S112). The random access response message includes the TA and uplink resource assignment. The UE 10 sends the handover complete message to the target eNB 20 (S114)
The LTE random access procedure may be either contention-based or contention-free. The random access preamble is contention-based when the UE 10 randomly chooses the preamble from among the available set of signatures. The random access preamble is contention-free when the UE 10 is assigned the signature for use by the eNB 20 via dedicated signaling.